Surface potential is one key variable in DG MOSFET compact modeling. A complete surface potential versus voltage equation and its continuous solution from the accumulation to strong inversion region are presented in this paper for undoped (lightly doped) symmetric double-gate (DG) MOSFETs. The results are based on the exact solution of Poisson’s equation with an amending mathematical condition of the continuity, allowing the surface potential to be accurately described from the accumulation region, through sub-threshold, finally to the strong inversion region. From presented surface potential solution, the dependence of the surface potential and corresponding gate capacitance on the quasi-Fermi-potential, silicon film thickness, gate oxide layer, and temperature are tested running different parameters, showing the solution continuity, smoothing, and high accuracy, compared with the 2-D numerical simulation. The presented equation and its solution will be useful in developing a complete surface potential-based DG-MOSFET compact model.